Architectures of DAS are known based on sending coherent light pulses into a sensing fiber and either measuring only the intensity of the coherent backscatter in a fiber (like Rayleigh scattering or backscatter from FBGs=fiber Bragg gratings), “DAS-I”, or determining also or only its optical phase, “DAS-P”. A change in both, the optical intensity and the optical phase, carries information about a change in the local strain of the fiber due to the change of optical path length to the scatter centers or to the fringes of an FBG. So tracking the intensity change and/or phase change allows to reconstruct to some degree an acoustic stimulus causing the strain variation over location and time in the fiber.
DAS-P has the advantage that any shift of fiber scatter centers due to strain linearly translates to a phase shift of the backscatter travelling back along the fiber, thus allowing better quantitative measurement of the strain event causing the phase shift. The phase of the optical backscatter cannot be directly measured due to the lack of detectors able to directly measure the electric field at optical frequencies, so DAS-P architectures employ a form of interference detection.
One way for phase detection is to split the backscatter from the fiber at a receiver side into two paths of different lengths, then combine both paths, thus bringing the coherent backscatter into interference with a temporally shifted version in the manner of an interferometer (like Mach-Zehnder or Michelson). The temporal shift corresponds to a location difference along the fiber, so in effect the backscatter form two different locations (along a “gauge length”) of the fiber do interfere, where the path length difference corresponds to about half the gauge length (taking into account two-way propagation).
Another way to derive the phase is to send double pulses into the fiber. The backscatter of both pulses travel back along the fiber, and the signal at any time on the detector corresponds to the interference of the backscatter from different fiber locations, where the location difference (gauge length) in this case corresponds to about half the pulse distance.
In both architectures the measured phase difference between two locations changes only if the optical distance between one location and the other changes by a different amount. The measured phase difference does not change if both locations shift by the same amount, e.g. if the fiber is stretched somewhere before the location, because this would increase the optical path length to both exemplary locations by the same amount.
Another way to derive the phase is to let the backscatter radiation reaching the instrument receiver path interfere with a local oscillator radiation (“LO”) which is (to at least some degree) coherent to the backscatter. Architectures are known to extract the backscatter phase information from that interference, like using optical I/Q demodulation (i.e. create two interferograms, where in the second the phase of the LO is shifted by π/2 with respect to the scatter signal) or by having LO and signal at slightly different frequencies allowing heterodyne detection from the beat signal.
The strain at any location is then derived by calculating the phase gradient, specifically the phase difference between two fiber locations of given distance (gauge length, e.g. 10 m).
The spatial resolution for the DAS strain measurement, i.e. the ability to discriminate adjacent acoustic events on the fiber or to exactly localize an event (e.g. expressed as its meter position along the fiber) depends on the DAS architecture and operating parameters. Depending on the architecture, the spatial resolution can be adjusted by different means. This may be by modifying the pulse parameters (like pulse length or double-pulse distance), the optical geometry (like arm length difference in an interferometer) or by changing the parameters in post-processing like selecting a suitable distance interval along the fiber for the phase difference calculation. Spatial resolution is generally connected to the instruments SNR (i.e. ability to measure/detect weak acoustic events or the maximum sensor length that can be utilized).
The spatial resolution can be adjusted with more or less effort or not at all by changing the measurement instrument setup or calculation parameters, depending on the architecture. Optimizing the spatial resolution for a certain application requires knowledge of the event to be detected (like strength or spatial distribution), so one is either limited to known or expected event properties or needs to re-process the data with a changed spatial resolution (where possible).
US 2012139746 A1 discloses the filtering of a signal from a plurality of distributed strain sensors. A subspace of a measurement space of the obtained signal is selected, wherein the subspace is characterized by a step having a selected step size. The document teaches averaging over time and space, wherein averaging ranges from Taylor development of measured signal (over time and over space) by estimating and minimizing the total error (random and systematic).
EP 2772738 A2 discloses a method for the detection of weak and noisy signals backscattered from a distributed fiber sensor. Therefore, the document discloses a threshold value setting circuit configured to set a threshold value in accordance with a change in an amount of noise overlapped with a measured signal. The method disclosed therein further teaches to switch between spatially averaged measured data and unfiltered data, depending on noise level (threshold).
WO 2015060981 A1 discloses the use of a dynamic window and an adaptive filter to reduce the noise in measured signals from a distributed optical fibre sensor.
U.S. Pat. No. 9,170,149 B2 discloses a distributed fiber optic sensor system with improved linearity, wherein a first optical signal is launched into an optical fiber resulting in a Rayleigh backscatter signal that is mixed with a second optical signal to generate mixed output signals. A phase detection and acquisition system determines a phase difference between first and second locations along the optical fiber based on phase information extracted from the mixed output signal and combines the phase information extracted from multiple acquisitions to detect strain on the optical fiber sensor. To measure vibration or strain, a distributed fiber optic sensing system may measure the amplitude and/or phase of a Raleigh backscatter signal returned from the fiber optic sensor when radiation of a narrowband optical source is injected into the fiber. This document uses a selectable pre-defined distance interval for calculating the phase difference between locations, but does not describe how to select the interval.
Noise from different sources (like laser noise, detector noise, shot noise . . . ) limit the achievable SNR which limits e.g. the minimum detectable acoustic event strength or the sensor reach (when the backscatter signal decreased below a useful level due to fiber attenuation). SNR can be improved e.g. by sending stronger pulses into the fiber, increasing the pulse rate, using low noise optical or electrical amplifiers, by reducing (worsening) the spatial resolution by using longer pulses or larger gauge lengths or by limiting further data processing to a reduced acoustic bandwidth (like filtering high and/or low frequency components). All of them have limits or undesirable side effects. Increasing the pulse power above some limit triggers non-linear effects in the fiber, in effect even reducing the useful pulse power after some distance. The noise level of optical or electrical amplifiers reach physical limits (like shot noise) and technological limits (like limited ability to filter-out EDFA spontaneous emission). Longer pulses and/or gauge lengths or performing some spatial data averaging in post-processing spoil the ability to resolve events that are closer than the gauge length and/or pulse length or spoil the linearity of the result. The maximum pulse rate is limited by the propagation time of a pulse forward and back along the fiber. If a next pulse is sent into the fiber before all radiation from the previous pulse has left the fiber, a disturbing mixing of signals happen.
When applying distributed acoustic sensing methods, the phenomenon called “fading” has been observed. Fading relates or is associated with optical properties of the fiber optical sensor along its length. In an undisturbed fiber, it is observed that the signal scattered back from a particular region of the fiber varies along the fiber. In a particular region of the fiber, scanner centers may be arranged relative to each other such that the backscattered light destructively interferes such that the detector detects a relatively low intensity. In other regions of the fiber, the intensity detected by the detector may be higher, since the destructive interference of the backscattered radiation is not as pronounced. Fading therefore negatively affects measurement intensities and also affects the strain along the fiber has derived from the detected intensities. In particular, there may be regions within the fiber from which very little or no light at all is backscattered. These regions may also be subjected to statistical variation, for example may change with environmental temperature or other external influences.
Thus, a limitation of the DAS architectures is “intensity fading” which is a coherent effect of the quasi-random distribution of Rayleigh scatter centers along the fiber. The backscatter from different scatter centers reaching the detector at the same time (being scattered from different parts of the coherent optical pulse) interfere in a quasi-random way (depending on the distribution of scatter centers) which can be more or less constructive or destructive. Therefore the interference from some parts of the fiber may lead to higher intensities and from other parts to lower intensities or even approach zero. The low signal leads to more noise or makes it impossible when the intensity is low. At such “faded” locations no phase or strain evaluation is then possible.
Conventional methods may not have addressed the problem of “fading” appropriately, for example not with respect to data processing. Instead, the conventional systems and methods have addressed fading by configuring the hardware, such as for example the light source, the parameters of the radiation injected into the fiber, the pulse pattern of the radiation injected into the fiber, the number and kind of detector used for detection of the backscattered radiation, multiple fiber couplers, the line width of the laser source, and so forth. These methods and systems may require to reconfigure and/or supplement the hardware to perform the strain measurements. This may be complex and cost-extensive. A number of publications address the issue how to avoid fading.
WO 2016205955 A1 discloses a fiber optic sensor shaped to have a frequency response that has less spectral fading than a sensor with a rectangular wrapping pattern.
US 2016191163 A1 discloses a fiber optic interferometer controller, wherein multiple detectors can be used to ensure that good quality signals are received along the entire fiber by using a combination of detectors that individually measure good quality signals only at limited locations along the sensing fiber. Multiple detectors, each responding to a single mode or a few modes, can eliminate signal fading.
WO 2016142695 A1 discloses a method for optical sensing using introduced reflection points at pre-determined positions in the fibre. The DAS signal processing can be optimized for use with this fibre by making use of the fact that now all sensing positions between each pair of reflection points measure the same signal. This means, for example, we can measure many positions between reflection points and then average the signals from these positions to improve the SNR. The frequency shift between first order sidebands 1202 and 1203 is proportional to the frequency modulation difference (f2−f1) whereas the frequency shift between second order sidebands 1204 and 1205 is proportional to 2(f2−f1). Therefore, the photo-detector output generates two beat signals, one of which is centered at (f2−f1) and the other at 2(f2−f1). Using a demodulator, the relative optical phase of the beat signals can be measured independently.
WO 2015170116 A1 discloses a fiber optic distributed sensing method that involves applying a predefined variation in frequency (LS) between the pulses of different interrogations and determining any variation in backscatter intensity arising from such predefined variation in frequency.
GB 2518767 A discloses a method and apparatus for optical sensing comprising circulators and multiple fibre couplers with different optical paths through the interferometers, Faraday-rotator mirrors and photodetectors.
US 2014176937 A1 discloses a distributed disturbance sensing device adopting OFDR, polarization controlling and analysis techniques, consisting of a ultra-narrow linewidth tunable laser source module, polarization generating and polarization splitting balanced detecting module.
CA 2854124 A1 discloses a phase sensitive coherent OTDR system including a frequency-shifting circuit to repeatedly translate the frequency of an optical pulse generated by a narrowband source to generate a train of interrogating pulses of multiple frequencies. The optical signals returned from the sensor arrangement in response to the pulse train is mixed on a photodetector with light from the narrowband source that has not been shifted to generate mixed output signals. The mixed output signals are filtered into frequency bands, and the phase for each frequency band is extracted.
GB 2515574 A discloses a distributed optical sensing method wherein a sensor is interrogated by the optical source transmitting a pair of optical pulses into the sensing fibre; and the optical receiver receives a returning composite optical signal, which is sampled and comprises light scattered from at least a region of a sensor.
GB 2489760 A discloses a distributed fibre optic sensing with a phase value based on a quality metric, wherein the processing of the backscatter data involves dividing the plurality of diversity samples into a plurality of processing channels and processing at least some of the channels to determine phase data for the channel.
US 2009122319 A1 discloses non-uniform sampling to extend range of interferometric sensors.
US 2002097636 A1 discloses a folded sensor array, wherein in order to overcome signal fading, a demodulation of the returned signal is required. The typical demodulation technique is the Phase-Generated Carrier (PGC) scheme, which requires a path-mismatched Mach-Zehnder interferometric sensor.
US 2017045410 A1 discloses a method for temperature sensing by strain measurement in an optical fibre. The document teaches that for single pulse systems this is not possible due to the well-known signal fading issue. A series of interrogations are launched into an optical fibre, each interrogation comprising interrogating radiation in at least one pulse pair, wherein the pulses of a pulse pair are introduced to the optical fibre with a time interval therebetween.
GB 2489749 A discloses Fibre Optic Distributed Sensing methods. As the backscatter signal exhibits a polarization dependence simultaneous or near simultaneous interrogation could help avoid problems with fading as the signals from both series may not fade at the same time. Two pulse pairs having different polarization states could be produced in a number of different ways. A single sample may be acquired for each analysis bin or multiple samples may be taken within an analysis bin and averaged together. To ensure that the maximum spatial resolution can be resolved it would therefore be necessary to sample such that the pulse pair had moved by about the gauge length between samples. By oversampling it is possible to generate diversity channels to overcome the problems of fading.
WO 2012030814 A2 discloses a distributed fiber optic sensing system, wherein the incoming backscattered signal is split into any two orthogonal polarization states and mixed each of these with a suitably aligned local oscillator signal. Using this approach has the benefits, that this arrangement avoids polarization fading (i.e., the weakening of the signal when the polarizations of the backscatter signal and LO signal are not the same).
The above mentioned publications propose to modify the measurement setup to address the issue how to avoid fading.